Evolving mobile cellular standards such as Global System for Mobile Communications (GSM) and Wideband Code Division Multiple Access (WCDMA) will likely require modulation techniques such as OFDM in order to deliver higher data rates. OFDM is a method for multiplexing signals which divides the available bandwidth into a series of frequencies known as sub-carriers.
In order to ensure a smooth migration from existing cellular systems to high capacity, high data rate systems using existing radio spectrum, new systems must be able to operate on a flexible bandwidth (BW). Third generation Long Term Evolution (3G LTE) has been proposed as a new flexible cellular system. 3G LTE is intended as an evolution of the 3G WCDMA standard. 3G LTE will likely use OFDM and operate on BWs spanning from 1.25 MHz to 20 MHz. Data rates of up to 100 Mb/s will be possible in the high BW 3G LTE service.
Low rate services as voice are also expected to use 3G LTE. Because 3G LTE is designed for Transmission Control Protocol/Internal Protocol (TCP/IP), voice over IP (VoIP) will likely be the service carrying speech. An important aspect of 3G LTE is its mobility, hence synchronization symbols and cell search procedures will be important due to the need for the UE to detect and synchronize with cells.
A cell search scheme that has been proposed for 3G LTE is as follows:
1. Detect new cell 5 millisecond (ms) timing using the primary synchronization channel (P-SCH).
2. Detect frame timing and cell group using the secondary synchronization channel (S-SCH). At this step, it is proposed that the number of transmit (TX) antennas used for synchronization channel (SCH) and primary broadcast channel (P-BCH) be detected. As used herein SCH includes, as the context requires, both P-SCH and S-SCH.
3. Detect the cell identification (ID) using the reference symbols (RS) (also referred to as Channel Quality Indicator [CQI] pilots).
Read P-BCH to receive cell specific system information. In step 2 above, the number of TX antennas for SCH and P-BCH transmission is yet to be determined. However, if SCH transmission using multiple TX antenna is implemented, the phase reference obtained from the SCH for equalizing the RSs before cell identification (ID) detection will be based on the sum of the radio channels from all TX antennas. This, in combination with the proposed RS design in 3G LTE, means cell ID detection will be non-trivial. The proposed RS design 100 for 3G LTE is seen in FIG. 1. FIG. 1 shows one resource block having 12 sub-carriers and 14 OFDM symbols, i.e. 1 ms=1 transmission time interval (TTI). As seen therein, the pattern is repetitive in frequency and time. More specifically, FIG. 1 illustrates the RS signal proposal using one virtual antenna 101, two virtual antennas 102 and four virtual antennas 103. In the multiple TX antenna cases 102, 103, the RS transmission from the different antennas are frequency and time multiple. Hence, assuming an RS transmitted from TX antenna i on sub-carrier j at time instant t, no transmission of data or RS are allowed on that position from the other TX antennas. Hence, when the user equipment (UE) uses certain RSs, only channel knowledge for that specific TX antenna is obtained, and hence equalizing using the SCHs as a phase reference, including the sum of the radio channels from all used TX antennas, will be erroneous.
What is desired is a method and apparatus which is adapted to perform robust cell ID detection in case of SCH transmission using multiple TX antennas.